1. Field of the Invention
This invention relates to a flyback transformer used for supplying high DC voltage to cathode ray tubes as in television sets, and more particularly to a flyback transformer with a high voltage variable resistor built therein for deriving focusing voltage, etc.
2. Description of the Prior Art
The cathode ray tube is supplied with a high DC voltage, as an anode voltage, which has been rectified after being elevated by a flyback transformer. Generally, this anode voltage is divided into medium-high voltages such as focusing voltage and screen voltage. As a voltage divider for obtaining these medium-high voltages, use has been made of a high voltage variable resistor.
Generally, in CRT display devices and high quality television sets required to provide distortionless clear images, it is essential that dynamic regulation be low with respect to a high voltage producing circuit using a flyback transformer. Dynamic regulation means voltage regulation in the duration of one scanning of the screen; if this dynamic regulation is high, distortion will be produced on the screen in a region where contrast is high. For this reason, it has been usual practice to connect a capacitor between the anode electrode of the cathode ray tube and the ground in order to compensate the cathode ray tube capacity so as to provide clear images.
Thus, a certain conventional type of high voltage producing circuit using a flyback transformer comprises the flyback transformer, a high voltage variable resistor, and a high voltage capacitor. An arrangement has already been proposed wherein a flyback transformer and a high voltage variable resistor are assembled as an integral component. In this case, the primary and secondary windings constituting the flyback transformer, and the resistor elements of the high voltage variable resistor are housed in a suitable insulating case, which is filled with insulating resin whereby the voltage-withstanding property of said parts housed in the insulating case is improved.
In the flyback transformer with a high voltage variable resistor built therein as described above, the arrangement for dividing the anode voltage which is a high voltage and deriving the focusing voltage and screen voltage which are medium-high voltages by means of the high voltage variable resistor makes it necessary to increase the resistance values of the resistor elements of the high voltage variable resistor. As a result, the impedance between the ground and the focusing voltage deriving section positioned intermediate between the resistor elements of the high voltage variable resistor becomes high, so that when an unnecessary radiation component due to ringing noise induced in the secondary winding of the flyback transformer rides on the resistor of the focusing voltage deriving section, it will not escape to ground, with the result that it is superposed on the focusing voltage, adversely affecting the screen. There is another form in which the secondary winding of the flyback transformer is divided into parts with a high voltage diode interposed between adjacent parts. In this case, however, an unnecessary radiation component due to noise during the switching of the high voltage diodes is also superposed on the focusing voltage, thus exerting an adverse influence on the screen.
The aforesaid arrangement wherein the primary and secondary windings of the flyback transformer and the resistor elements of the high voltage variable resistor are integrally built in the insulating case has advantages that desired electrical connection between the flyback transformer and the high voltage variable resistor can be made within the insulating case whereby simple wiring suffices in spite of high voltage. There is another advantage that as a whole the arrangement can be reduced in size as compared with an instance in which the flyback transformer and the high voltage variable resistor are constructed as separate parts. However, it is desired to eliminate the aforesaid drawbacks while enjoying these advantages.